Thermal shock-resistant ceramic article

ABSTRACT

A thermal shock-resistant ceramic article and a method of manufacturing the article include alternating layers of a first material ( 2 ) comprising a fusible, particulate ceramic composition and a second material ( 3 ), typically comprising a porous, pyrolyzable material. The layered structure increases the article&#39;s work of fracture and toughness, and may lead to improved thermal shock-resistance. The method advantageously uses a sheet, film or sleeve to prepare the article for firing. The composition, thickness, and porosity of the second material ( 3 ) will affect the desired properties. The method is particularly adapted for manufacturing cylindrical articles, including stopper rods, nozzles, and pouring tubes for the metal casting industry.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Non-Provisionalapplication No. 09/647,672 filed Nov. 17, 2000, now U.S. Pat. No.6,395,396, which is 371 of PCT/BE99/00041 filed Mar. 25, 1999, which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a ceramic article and a method for theproduction of the article, and more particularly to an article andmethod comprising alternating layers of dissimilar materials to producean article with an improved work of fracture.

2. Description of the Prior Art

Ceramic articles are, of course, well known and find many commercialuses where, for example, hardness, refractory properties or relativechemical inertness are desired. A serious deficiency of ceramicproducts, however, is their brittleness or, stated in other words, theirpoor work of fracture or toughness. This limitation has hindered theentry of ceramics into those areas where their other properties would behighly desirable, for example, U.S. Pat. Nos. 5,657,729 and 5,687,787describe attempts to incorporate toughened ceramic parts into internalcombustion engines.

Brittle materials typically fail catastrophically and often withoutwarning. Conversely, tough materials will normally bend or deform beforefailure. In most applications, the latter type of failure is preferred.Common methods of testing toughness are a Single Edge Notch Bend (SENB)test and a Modulus of Rupture (MOR) test. Both involve a three pointbending geometry and differ in the presence or absence, respectively, ofa notch in the sample to be tested. In both, a stress on a sample isslowly increased as a function of strain. The resultant area beneath aplot of stress versus strain is the work of fracture and represents theamount of energy absorbed during one of these tests.

A tougher material has the ability to absorb greater amounts of energythan a more brittle material. One way a material may absorb energy is bymicroscopic morphological changes. For example, tough metals or alloyslike steel absorb energy by, for example, developing dislocations,slipping across crystal planes, or undergoing crystal twinning. Amaterial may also absorb energy by creating new surface area through aprocess known as crack blunting. For example, composite materials, suchas fiberglass, are heterogeneous and contain a plurality of phases. Whena crack reaches a phase boundary, the crack may propagate along theboundary, and create a delamination crack. In effect, the crack isblunted at the phase boundary. Blunting reduces crack propagation byspreading the energy at the crack tip over a larger area.

Generally, ceramic materials cannot absorb much energy because theircrystal structure resists microscopic morphological changes.Additionally, crack blunting does not occur to any substantial extent inhomogeneous materials. Attempts to improve the toughness of ceramicshave concentrated on introducing some degree of heterogeneity into theceramic. For example, an increase in toughness has been accomplished byproviding a second phase within the ceramic, such as a layer of fibers;see, e.g., U.S. Pat. No. 5,589,115. Presumably, the fiber layer disruptscrack propagation by blunting the crack tip. Unfortunately, thissolution is not without its flaws. The green ceramic matrix, in whichthe fiber is placed, shrinks when fired, but the fiber itself does not.This results in delamination of the fiber from the ceramic and createswhat are essentially voids in the brittle ceramic. Voids normally act toconcentrate stresses, initiate crack formation, and increase thelikelihood of brittle failure.

Techniques to overcome this problem involve a plurality of mats ofceramic fibers impregnated with a particulate ceramic material, liquiddiluent and organic binder. This technique places the ceramicparticulate in more intimate contact with the fiber. During firing,however, the ceramic particulate still shrinks. While an improvementover the prior art, this method does not completely overcome thedelamination problem, and results in a ceramic composition with variablemechanical properties.

Delamination can be substantially overcome by a technique involving meltinfiltration. This technique involves perfusing a molten ceramicmaterial into ceramic fibers. Although delamination is reduced, severalnew problems arise. Very high temperatures are required to melt ceramicsand some ceramics sublime before they melt. The high temperatures canalso damage the ceramic fiber. Even if the ceramic can be melted, theviscosity of a molten ceramic is so high that the rate of infiltrationinto the fibers is very slow and the molten ceramic may nothomogeneously wet the surface of the fibers.

The extremely high temperatures of melt infiltration can be avoided by avapor infiltration technique, see, e.g., U.S. Pat. No. 5,488,017. Atrelatively low temperatures, a vapor comprising a ceramic precursorinfiltrates ceramic fibers. Later the chemical is decomposed to leave aceramic residue. For example, gaseous methyltrichlorosilane may bedeposited onto ceramic fiber at just several hundred degrees centigradeand later decomposed to silicon carbide at a temperature, which may beless than 12000 C. A silicon carbide matrix is created, which isreinforced by the ceramic fiber. Although overcoming some of thedisadvantages of previous processes, vapor infiltration is verytime-consuming and limited to ceramics with volatile precursors.

U.S. Pat. No. 5,591,287 avoids using fibers, melts or volatileprecursors. This patent creates one or more zones of weakness betweenlayers of sinterable, particulate ceramic material. The zones ofweakness consist of very thin layers of non-sinterable or weaklysinterable material. Examples of a non-sinterable material includecarbon or an organic polymeric material, which may pyrolyze into carbon.A weakly sinterable material may form bonds with itself and thesinterable, particulate ceramics, but the bonds so formed should besubstantially weaker than the bonds formed within and between thesinterable ceramic layers.

The zones of weakness should be less than about 50 microns to permitsintering between ceramic layers. Such thin zones of weakness may becreated by spreading a suspension of non-sinterable or weakly sinterablematerial over one surface of a preformed, sinterable ceramic. Many zonesof weakness may be produced by depositing the non-sinterable materialbetween each of a plurality of ceramic layers. The resulting zones ofweakness may deflect cracks propagating through the ceramic. The crackmay then travel along the zone of weakness and form a delamination crackbetween the layers of ceramic. The process of delamination increases thework of fracture. Unfortunately, this method is limited to sinterableceramic materials that have been preformed into a layer over which anon-sinterable material can be spread. This restricts both thecomposition and the geometry of articles, which may be made using thismethod.

Despite these known methods for improving the toughness of ceramicarticles, there is still a need in the industry for a method to producequickly and cheaply a tough morphology in a commercially useful shape.Simply mixing a ceramic fiber into a sinterable ceramic often leads todelamination between the two materials. Methods to prevent delaminationsare either too time-consuming, limit article geometry or composition,produce inconsistent results, or require excessive temperatures. Acommercially viable method is needed to toughen a ceramic article.

SUMMARY OF THE INVENTION

The present invention relates to a multilayer ceramic article and amethod of making the same. In a broad aspect, the article comprises aplurality of layers of a first phase comprising a fused and/or carbonbonded particulate ceramic; and, disposed between adjacent layers offirst phase a layer of a mechanically or chemically different secondphase. The article of the present invention is depicted as possessingsubstantially improved work of fracture compared to a ceramic articlewithout a layered structure.

The first phase is described as a fused or carbon-bonded, particulateceramic. The second phase may be a porous material, such as a metalmesh, or a weakly fused or carbon-bonded refractory, or may evencomprise the pyrolyzed residue of a combustible material.

Alternatively, the second phase may be fused by a process independent ofthe first phase, such as by sintering if the first phase is acarbon-bonded ceramic. In other embodiments, the second phase may sharea similar bonding mechanism with the first phase but will possesssignificantly weaker mechanical properties.

The invention describes layers of the first phase as preferentiallyhaving a thickness from between about 0.5 mm to about 10 mm with layersof the second phase having a thickness from about 0.005 mm to about 2mm.

One aspect of the invention describes the layers as spiralling along thelongitudinal axis of a cylindrical shape. The article may also comprisea bore.

The present invention also relates to a method for producing a ceramicarticle having improved thermal shock-resistance and toughness. In abroad aspect, the invention describes a method to fashion a ceramicarticle by alternating layers of a first material with a secondmaterial. The first material may be a fusible or carbon-bonded,particulate ceramic. The second material is expected to form a weaklyfused or weakly carbon-bonded layer.

Alternatively, the second material may fuse by way of a processindependent of the first material, such as by sintering if the firstmaterial is a carbon-bonded ceramic. The second material may beproffered as a sheet, film, membrane, or even a casing onto or intowhich the first material may be placed. The layers are then pressed intoa piece and fired to form the finished article.

In one aspect of the invention, the second material is described as acombustible material, which may pyrolyze at elevated temperatures. Thecombustible material may be an organic material, such as plastic, paper,cotton or other natural or synthetic polymer.

Still another aspect of the invention describes a process to make alayered, cylindrical article. The first material is described as aceramic refractory and the second material may be a combustible sheet.Layers are alternated by coating the combustible sheet with the ceramic,compacting the ceramic on the sheet, and subsequently rolling the coatedsheet onto itself thereby creating a cylinder with a “jelly roll”morphology. The second material may alternatively be a tubular casing.The ceramic material may then be inserted into the casing, compacted,and formed into any desired shape, including a “jelly roll.”

A still further aspect of the invention describes a method of making thearticle into a tube by wrapping a coated sheet or filled casing around amandrel, pressing the wrapped sheet or casing on the mandrel, removingthe piece from the mandrel, whereby a bore is created where the mandrelhad been, and firing the wrapped sheet or casing to make the article.

Other details, objects and advantages of the invention will becomeapparent as the following description of a present preferred method ofpractising the invention proceeds.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of an article having the alternating layeredstructure of the invention.

FIG. 2 depicts a method for creating the article of FIG. 1 using anorganic sheet.

FIG. 3 shows a method for creating the article of FIG. 1 using anorganic casing.

In FIG. 1, an article fashioned into a tube by the method of theinvention is shown. The tube 1 comprises a plurality of alternatinglayers of a first phase 2 and a second phase 3. The total number oflayers depends upon the thickness of each layer and the desiredthickness 4 of the tube 1. Both the first phase 2 and the second phase 3spiral outward from a bore 5 within the tube 1. Such a geometry inhibitsa crack 6 from propagating perpendicularly to the longitudinal axis 7 ofthe tube 1.

In FIG. 2, a method of making a tubular ceramic article is illustrated.An organic sheet 10, which is comprised of a second material, is unwoundfrom a take-off roll 11. A first material 12 is deposited on the sheet10, and the sheet 10 is wound on a mandrel 13 to form a tube 14 having aplurality of layers until the desired thickness 15 is achieved.

In FIG. 3, an alternate method involving a casing 20, which comprises asecond material is depicted. A first material 21 is feed into a hopper22 and forced into the casing 20. The filled casing 20 is compactedbetween rollers 23 and wound up on a mandrel 24 to form a tube 25.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention describes a ceramic article, which has improvedthermal shock-resistance and toughness, and a method of producing thesame. The article is especially useful in the continuous casting ofmolten metals, and may be manufactured so that different ceramiccompositions are exposed to the molten metal stream, slag line, andmould area. The method comprises depositing and compacting a firstmaterial onto or into a second material. The substrate may then beshaped, pressed, and fired into a ceramic article.

The article comprises a plurality of layers comprising at least twophases. Adjoining layers of a first phase are in physical contact witheach other, and between layers is an interface. The interface comprisesan area of reduced contact between adjacent layers of a first phase,whereby a propagating crack may preferentially follow the interface andeffectively blunt the crack. Crack blunting increases the energyabsorbed by the article, as measured by work of fracture, and improvesthe article's toughness.

The first phase may comprise any type of fusible or carbon-bonded,particulate ceramic. For convenience, “fused” or “fusible” is meant toinclude those ceramics, which may be “sintered” or “carbon-bonded” . Aparticulate ceramic comprises any type of ceramic whether powdered,granular, fibrous, chunked, or any shape or combination of shapes, andof whatever size, which are amenable to being pressed into a form.Fusible implies a ceramic, which may be fired to create a fused articleout of a collection of ceramic particles. A binder is often used to holdtogether a fusible ceramic before firing. The ceramic is ultimatelyfired at a temperature high enough to coalesce the ceramic particles,thereby creating a fused mass. A certain amount of void fraction mayremain because the ceramic particles do not completely fuse or losetheir individual identity. In contrast, a non-fusible ceramic maysublime or degrade before fusion occurs, or the selected firingtemperature may simply be too low to trigger fusion. The fusible,particulate ceramic may be selected from any number of commonly knownceramic compounds, and will usually, at least in commercialapplications, comprise a mixture of ceramic compounds. The actualmixture chosen will depend upon the particular application in which theceramic article will be used. For example, ceramic compositions, whichare used in handling, molten metals may comprise alumina, silica,silicon carbide, zirconia, and other refractory ceramic compounds. Atypical refractory ceramic mixture used in stopper rods for steelprocessing may comprise major amounts of alumina and graphite with minoramounts of silica and other refractory ceramics. Graphite, a non-ceramicparticulate material, is commonly added to improve thermalshock-resistance. Alternatively, a specialized refractory havingexcellent corrosion and erosion resistance, but poor thermalshock-resistance, may comprise a major amount of alumina with minoramounts of zirconia and silica.

The invention also creates the possibility of using new ceramiccompositions, which take advantage of the improved toughness of alayered morphology. For example, ceramic compositions may be used thatpreviously had been too brittle or thermal shock-sensitive but possessedotherwise desirable properties. Ingredients, which had been necessaryfor certain physical properties, may be reduced or eliminated. Inparticular, graphite, which improves thermal shock-resistance, undergoesdeleterious oxidation. A layered morphology may enable the use of lessgraphite, resulting in a product less sensitive to oxidativedegradation.

The invention is not limited to using only a single ceramic mixture orcomposition within any one article. In fact, it is contemplated that aplurality of ceramic compositions will be used in any finished article.This may be particularly advantageous when different properties aredesired at different places of the finished article. For example, insubentry shrouds for the continuous casting of molten metals, a firstceramic composition having good slag resistance may occupy an outerlayer of the shroud, a middle layer may comprise a ceramic compositionhaving good thermal shock-resistance, and an inner composition maycomprise a good erosion-resistant ceramic.

Along with a first phase comprising a ceramic material, the article alsohas a second phase. The second phase separates and may sandwich layersof the first phase. The second phase may comprise, for example, carbonfibers, a metallic mesh, a pyrolyzed residue, a relatively weakly fusedceramic, or a ceramic fused by mechanism different than the mechanism ofthe first phase. In all cases, the second phase is intended to interferewith inter-layer fusion of adjacent layers of first phase. Suchinterference creates an interface, which is weaker than the first phase.The interface is characterized as a region comprising relatively fewbonds between adjacent layers or as a discontinuity in the article'smicrostructure. The second phase may be introduced as a powder, slurryor suspension but, preferably, the second phase begins as a substratecapable of supporting or containing the ceramic particulate. Mostcommonly, the substrate will be a sheet or casing. The term “sheet” ismeant to include any film, textile, cloth, or any other like substancecharacterized by two of its dimensions greatly exceeding its third.“Casing” is meant to include any flexible sheath, jacket, tube, sleeveor similar article, which may be formed by connecting opposing edges ofa sheet, and into which the ceramic particulate may be placed.

A sheet or casing will most commonly be an organic material, such as asynthetic or natural polymer, but may also include a mesh made Erom aninorganic material. Inorganic materials include metal or inorganicfibers such as graphite or ceramic fiber. Synthetic polymers include,for example, polyolefins or polyesters, but may include any type ofsynthetic polymer that may be fabricated into sheet or casing. Naturalpolymers include, for example, paper or cotton, but other naturalpolymers may also be used.

A sheet is preferably a paper product, owing mostly to paper's low cost,good mechanical strength, and low stretching under tension. The sheet islikely to experience tension during processing, and many commonsynthetic polymers stretch unacceptably. The thickness of the sheet isroughly dependent on the thickness of the ceramic layer. A thicker sheetis preferred to support a thicker ceramic layer. The sheet willgenerally be thinner than the ceramic layer and often will be aboutone-tenth the thickness of the ceramic layer. It is appreciated,however, that the invention incorporates a range of thicknesses at leastbetween about 0.005 mm to about 2.0 mm, irrespective of the thickness ofthe ceramic layer.

Typically, the sheet, especially organic sheets containing oxygen aspart of their chemical composition, will pyrolyze at temperatures neededto fuse the ceramic material. Pyrolysis may leave a trace residuebetween adjacent ceramic layers, but may also leave a defect, which isweaker than the rest of the fused article. The defect may be describedas a weakly fused region relative to the fusion found in the ceramiclayers. A crack propagating within a ceramic layer may impact thisregion and deflect along the defect thereby forming a delaminationcrack. The energy needed to produce the delamination increases the workof fracture and, correspondingly, the toughness of the ceramic article.

The combustible sheet will preferably have holes. The holes shouldpermit adjacent layers of the ceramic particulate to contact each otherthrough the holes in the sheet. Upon firing of the article, contactbetween the ceramic layers through the holes may permit some fusionbetween layers. The combustible sheet is expected to pyrolyze at firingtemperatures but not before preventing substantial contact and,therefore, fusion in the region between ceramic layers. The region,which had been occupied by the now pyrolyzed sheet, may after firingcontain a weakly fused defect in the ceramic article.

It will be understood that, even in the absence of holes in thecombustible sheet, some fusion may occur between ceramic layers. Holes,however, may permit the sheet to be thicker and, consequently, strongerand easier to handle than sheets without holes. Weakly fused defects maybe produced by sheets without holes, but these sheets may need to bethinner than corresponding porous sheets. A thinner sheet could lead tomanufacturing difficulties when producing a ceramic article according tothe method of this invention. Thinner sheets are expected to flex moreand support less ceramic particulate before buckling.

A sheet without holes or a sheet of excessive thickness may even createdefects in the ceramic article that actually decrease toughness. Thesedefects may result from little if any fusion between ceramic layersafter the combustible sheet has pyrolyzed. A crack propagating through aceramic may encounter a defect, which had been created between ceramiclayers by pyrolysis of a combustible layer. The crack may deflect alongthe plane of the defect. Without some fusion between the ceramic layers,the crack will propagate rapidly along the plane of the defect becauseno additional energy will be needed, for example, to break bonds formedby fusion. Toughness will generally not be improved by this type ofdefect because, as previously taught, greater toughness correlates withgreater energy input. Cracking without the need for energy input wouldnot be expected to improve toughness.

A competition exists, therefore, between maximizing and minimizing thedegree of fusion between ceramic layers. Less fusion between ceramiclayers creates a more “perfect” defect, and may increase the chance thata crack propagating through the ceramic will deflect along the plane ofthe defect. Once the crack has deflected along the defect, however, itmay then be desirable to have as many points of fusion as possiblebecause more energy would be needed to break the bonds. But, the greaterthe degree of fusion between ceramic layers, the more the defect beginsto look like the ceramic matrix and the less chance the crack willdeflect along the defect. The number, shape, and size of holes, as wellas the sheet thickness, will affect the degree of fusion in the article;therefore, the combustible sheet should be selected with this balance inmind.

The combustible sheet will preferably be a porous paper with a thicknessabout 0.005 mm to about 0.5 mm. A porous paper is a paper, which permitslayers of fusible, particulate ceramics on either side of the paper tocontact one another intermittently. Porous paper may include thosepapers having holes, which are similar to or larger than the size of theceramic particulate. Such holes may, for example, be defined by spacesbetween cellulose fibers making up the paper. The holes may also becreated by mechanical means, such as by perforating the paper. Paperpossesses a substantial amount of rigidity and strength, which is neededto support the ceramic material in the method of the invention. At thesame time, the paper may be made thin enough to permit intermittentcontact between ceramic layers on either side of the paper. Paper alsohas a relatively low flash point and leaves minimal pyrolyzed residue.

The combustible sheet may also be a polymer film, such as polypropylene,polyethylene or any flexible organic polymer sheet. Plastic filmsnormally will be contiguous and free of pinhole defects. This propertymay inhibit fusion between ceramic layers; although, holes may be madein the plastic to improve fusion between ceramic layers. Polymer filmsdisadvantageously may stretch when under tension, as may be experiencedby the film during processing.

Holes in a combustible sheet permit the sheet to be substantiallythicker than without holes. For example, porous paper sheets over 1.0 mmthick may still permit adjacent ceramic layers to contact one anotherand fuse together when fired. Handling includes all those processesconcerned with the sheet itself, for example, rolling or unrolling thesheet, and also all those processes related to the sheet in combinationwith the ceramic. By comparison, nonporous sheets should besubstantially thinner to achieve some fusion between ceramic layers. Asa sheet becomes thinner, the sheet becomes increasingly flexible andsubject to stretching. These properties make the sheet more difficult tohandle.

Mechanical properties of the sheet are important because the inventionutilizes the sheet as a support in the process. In one embodiment, theceramic article is cylindrical as, for example, a nozzle, pouring tubeor stopper rod to be used in molten metal processing. A combustibleorganic sheet is unwound from a take-off roll and transportedhorizontally towards a take-up roll. Between the two rolls, the sheet iscovered with a fusible, particulate ceramic to a thickness between about0.5 mm and about 10 mm. During the process, the composition andthickness of the ceramic layer may be changed one or more times. Thesheet will have a thickness equal to at least about one-tenth thethickness of the ceramic layer. Thinner sheets may also be used if thesheets' mechanical strength is sufficient. Thicker sheets may also beused if desirable. Preferably, the sheet has a thickness between about0.05 mm and 1.0 mm.

After being deposited on the sheet, the ceramic material is thencompacted to increase the density of the ceramic layer. The ceramiclayer should be compacted enough to permit easy handling but shouldstill be flexible enough to be bent without cracking. The sheet with thecompacted, fusible ceramic is wound up on the take-up roll. When thedesired thickness on the take-up roll is achieved, the take-up roll isremoved. Material on the take-up roll may comprise the ceramic articleor the material may be rewrapped into another shape or around anotherceramic piece. In this fashion, spirals of layers of sheet and ceramicmaterial are deposited within the ceramic article.

Rewrapping the compacted ceramic/sheet permits a second compactedceramic/sheet to be co-wrapped with the first. In this fashion, twosubstantially different ceramic compositions may be intimately fused toform the finished article. For example, a good thermal shock-resistantceramic may be layered with a good erosion-resistant ceramic inalternating layers. The finished article may gain the benefits of goodthermal shock-resistance and good erosion-resistance. In a like fashion,a third, fourth or more ceramic compositions may be co-wrapped toachieve optimal properties.

After being formed into its final shape, the wrapped roll is pressedinto a piece. Pressing can utilize any number of known processes, forexample, as is common in three-dimensional objects, isostatic pressingmay be used. The piece is then fired at a temperature necessary forfusion. Of course, firing temperature depends on the ceramiccomposition. Firing temperature may also depend on several otherfactors, such as firing time and desired porosity in the finishedarticle. Such parameters are well known by those skilled in the art.After firing, the finished ceramic article results.

Although an article of this invention may be produced using sheet, thepreferred method of producing the disclosed article comprises placing aceramic particulate into a casing and compacting the filled casing.Techniques used in the sheet process may also be applied when using acasing. Unlike compacting on a sheet, the compacted casing presents aneasy way to manipulate the ceramic particulate because the ceramicparticulate is completely contained within the casing. By comparison,compacted ceramic on the surface of a sheet could fall from the sheet ifturned upside down or even sideways. Filling the casing with ceramicparticulate normally involves a technique similar to sausage making, inthat the ceramic is placed into a hopper and forced into a casing. Thefilled casing is compacted, and the compacted casing may be manipulatedin any manner to fashion an article. Conveniently, the casing iscompacted between a pair of rollers, but a single roll may be preferredin certain circumstances. Naturally, the type of ceramic particulatebeing fed into the casing at any one time may vary depending on the typeof article being made and the properties required. For example, athermal shock-resistant ceramic may be used at one stage of casingfilling, while a more erosion-resistant composition may be used during alater stage. Several casings having different ceramic compositions mayeven be co-wrapped or copressed and fired into the finished article.

As described, the casing may be any type of tubular material such aspaper or synthetic tubing, but in practice, the preferred material willbe a cotton gauze. Cotton gauze is inexpensive, readily available,pyrolyzable, and is comprised of a very open weave containing manyholes. The gauze may be placed under tension to completely open theweave. The gauze may also be impregnated with binder, graphite, or anyother substance shown beneficial to the process. It is useful to notethat, unlike sheet-produced articles, an article produced by winding upa compressed casing will have a morphology characterized by a layer ofceramic followed by two layers of the casing. Practically, the twodistinct layers may be considered as one. The compacted casing may belikened to a sheet; therefore, it is anticipated that a second or eventhird layer of ceramic particulate may be placed on the outer surfacesof the compacted casing. In either embodiment, the layers within afinished article may be other than strict alternating layers ofceramic/substrate expected in a sheet-produced article.

Whether using the sheet or casing process, the ceramic article producedwill commonly be cylindrical, and may also comprise a bore. Nozzles andpouring tubes will naturally contain a bore. A bore may be easilyfashioned in the finished article by winding the coated sheet or filledcasing around a mandrel. Pressing and firing win then create a ceramicarticle with a bore. Layers comprising the first and second materialswill spiral outwardly from and around the bore; although, this spiralneed not be concentric and may even be interrupted by other componentswithin the article or by the required shape of the finished article.

The process is not limited to the creation of cylindrical articles.Various other shapes may also be formed. In articles produced usingsheet, the sheet should be at least about 0.005 mm. to about 0.5 mm;although, thinner or thicker sheets may be used depending on processingconditions. Additionally, the ceramic is not necessarily compactedbefore pressing. For example, a method to produce a simple board productmay comprise laying down a sheet, placing a particulate, fusible ceramicmaterial on the sheet, laying down a second sheet and a second layer ofceramic material, and continuing to alternate layers until the desiredthickness is achieved. Such a process is also useful in manufacturingslide gate plates. The entire article may then be pressed and fired toform the multilayer article. Manufacturing using the casing method maybe even more versatile than manufacturing with sheet material. Thecasing holds the ceramic in place and, consequently, may be positionedwith greater efficacy.

The layered article may be embedded or even fully encapsulated in anon-layered object. This may be especially useful to arrest cracking atparticular points of a commercial product. For example, a sub-entryshroud used in the continuous casting of steel will experience extremethermal stress, chemical assault and erosion at the slag line.

Inclusion of a layered article within the shroud at the slag line mayeffectively arrest cracks and permit the use of more erosion-resistantceramics.

EXAMPLE I

A quantity of porous paper of thickness 0.05 mm was removed from a rollof paper. The paper was cut to a predetermined length and flattened. Astandard mix of a fusible, particulate ceramic composition was depositedonto the paper. The mix comprised 50-55 weight percent alumina, 13-17weight percent silica, and 30-35 weight percent graphite. Thecomposition was selected as representative of the type of ceramic mixused in nozzles for the continuous casting of molten steel. The ceramicmix on the paper was compacted to a thickness of 1.0 mm, and the coatedpaper was then continuously wrapped around a steel mandrel until adesired thickness was achieved. The coated paper on the mandrel wasisostatically pressed to compact the ceramic particles, thereby forminga piece. The piece was fired at a temperature of up to 1000 C. in areducing atmosphere to form a ceramic article. The ceramic article wascut into test samples for Modulus of Rupture (MOR) tests. A comparative,non-layered standard was created consisting of the ceramic mix withoutthe paper sheet. The same ceramic composition, pressing and firingconditions were used as for the layered piece. Ten samples of thenon-layered piece were also cut for MOR tests. The multilayer piece hadan average work of fracture equal to 177,000 erg/cm² compared to thestandard piece that had an average work of fracture of only 42,000ergs/cm².

EXAMPLE II

A tubular article of the present invention was made by feeding aparticulate ceramic mix into a first open end of a hopper. The sameceramic composition was used as in the preceding example. A medicalgrade cotton gauze sleeve was placed over a second open end of thehopper. The ceramic was extruded from the hopper into the cotton sleeve.The sleeve was drawn between two rollers whereby the ceramic mix insidethe sleeve was compacted. The compacted sleeve was wrapped around amandrel and shaped into a cylinder. The wrapped sleeve was isostaticallypressed at up to 140 MPa (20,000 psi) and fired below 1000 C. in areducing atmosphere.

EXAMPLE III

Particulate alumina-graphite was compressed inside a cotton gauze sleeveand formed into an annular ring having twelve layers ofalumina-graphite. Each layer was less than 5 mm thick. A sub-entryshroud was created with the annular ring at the slag line and completelyencapsulated by the body of the shroud. The shroud was placed intomolten steel at 2900° F. to the level of the annular ring. Afterreaching temperature, the shroud was removed and sprayed with water tosimulate extreme thermal shock conditions. The exterior of the shroudcracked at the level of the annular ring. After sawing the shroudlongitudinally, the crack was clearly seen to begin at the exterior ofthe shroud and to stop at the multi-layer annular ring. In a similarshroud without the annular ring, the crack extended completely throughthe shroud. The annular ring, which was made from a layered material,was deemed capable of blunting the advancing crack tip.

What is claimed:
 1. A method for producing an article for use withmolten metal, comprising: a) filling a flexible sleeve with a fusible,particulate ceramic composition; b) compacting the filled sleeve; c)layering a plurality of compacted sleeves to form a piece; d) pressingthe piece; and e) firing the piece at a temperature sufficient to fusethe ceramic composition.
 2. The method of claim 1, wherein the sleevecomprises a combustible material.
 3. The method of claim 2, thecombustible material is comprised of a material selected from the groupconsisting of natural and synthetic polymers.
 4. The method of claim 1,wherein the sleeve is porous.
 5. The method of claim 1, wherein pressingis accomplished by isostatic pressing.
 6. The method of the compactedsleeve has a thickness between about 0.05 mm and about 10 mm.
 7. Themethod of claim 1, wherein the sleeve is sequentially filled with atleast two different ceramic compositions.
 8. The method of claim 1,wherein layering comprises rolling the compacted sleeve on itself. 9.The method of claim 1, wherein layering comprises rolling the compactedsleeve around a mandrel.
 10. A method for producing an article for usewith molten metal, comprising: a) filling a flexible, porous sleeve witha fusible, particulate ceramic composition; b) compacting the filledsleeve; c) wrapping the filled sleeve around a mandrel to form a piece;d) isostatically pressing the piece; and e) firing the piece at atemperature sufficient to fuse the ceramic composition.
 11. A method ofmaking a refractory article having a plurality of layers and adapted forchanneling a stream of molten metal, the method comprising: a) producinga first layer comprising a first fusible, particulate ceramiccomposition having a thickness between about 0.05 mm and about 20 mm anda porous sheet having a thickness between about 0.005 mm and about 2.0mm; b) depositing onto the porous sheet a second layer comprising asecond fusible, particulate ceramic composition having a thicknessbetween about 0.05 mm and about 20 mm; c) pressing the layers togetherso that the first fusible, particulate ceramic composition contacts thesecond fusible, particulate ceramic composition through the poroussheet, thereby forming a piece; and d) firing the piece at a temperaturesufficient to fuse the first and second ceramic compositions, therebymaking the article.
 12. The method of claim 11, wherein the methodincludes compacting the first layer before depositing the second layer.13. The method of claim 11, wherein the first ceramic compositiondiffers from the second ceramic composition.
 14. The method of claim 11,wherein the sheet comprises a combustible material.
 15. A method forproducing a refractory article having a plurality of layers and adaptedfor channeling a stream of molten metal, comprising: a) producing alayer comprising a first, fusible, particulate ceramic compositionhaving a thickness between about 0.05 and about 10 mm and a porous sheethaving a thickness between about 0.005 mm and about 1 mm b) compactingat least a portion of the layer; c) spiraling the layer about a centralaxis, whereby a plurality of spiraled layers spiral outward from thecentral axis d) pressing the spiraled layers so that the ceramiccomposition on a spiraled layer contacts the ceramic composition of anadjacent spiraled layer through the porous sheet, thereby forming apiece; and e) firing the piece at a temperature sufficient to fuse theceramic compositions, thereby making the article.
 16. The method ofclaim 15, wherein the central axis is occupied by a mandrel.
 17. Themethod of claim 15, wherein pressing is accomplished by isostaticpressing.
 18. The method of claim 15, wherein the a first spiraled layerincludes a first ceramic composition and a second spiraled layerincludes a second ceramic composition.
 19. The method of claim 15,wherein the sheet comprises a combustible material.